(1) Field of the Invention
The present invention generally relates to a following servo control apparatus, such as a servo mechanism for positioning heads of a magnetic disk apparatus. More particularly, the present invention is concerned with a settling monitor system which detects an event that a predetermined settling condition has been satisfied after a transient state.
(2) Description of the Related Art
Recently, an effort has been made to reduce the space between concentric recording tracks in order to increase the recording density, and it has been required that heads be positioned more precisely.
Generally, a disk storage apparatus is used as a data storage device. Thus, it is necessary to prevent data recorded on a track from being erased due to erroneous head positioning, and from being magnetically weakened. For these requirements, in a normal write mode, a head position signal showing a quantity to be controlled is monitored, and an upper device, such as a computer, is notified of a following control error if the level of the head position signal exceeds a predetermined threshold value.
Meanwhile, the head is moved in a seek operation in order to access requested data. The seek operation ends when the head movement has been completed so as to position the head on a desired track position, and a transient response has been settled in the positioned state. After the completion of the seek operation, the occurrence of a following control error is not permitted because data writing starts from this time.
As described above, the magnetic disk apparatus needs the settling monitor procedure for the following control after the completion of the seek operation. A settling decision reference signal is set to a value smaller than an off-track value which does not reduce a reproduction signal level of data recorded on an adjacent track. That is, the settling decision reference signal is selected so that it has a margin. When the transient response is continuously within a range defined by the settling decision reference signal for a predetermined settling monitor time, it is determined that the settling has been completed. In order to decrease the error rate, a long settling monitor time is needed, and hence the access time is lengthened. Hence, it is desired that a desired or target error rate can be obtained by a settling monitor time as short as possible.
FIG. 1 is a block diagram of a magnetic disk apparatus equipped with a conventional settling monitor system. A magnetic head 102 of a magnetic disk apparatus 100 reads a position information signal from a magnetic disk. A position detector 104 amplifies and demodulates the position information signal from the magnetic head 102, and generates a position error signal PE indicating a deviation from a target track. A servo MPU (MicroProcessor Unit) 108 executes a digital servo process, and reads a digital signal obtained by converting the position error signal PE from the position detector 104 by means of an AD (Analog to Digital) converter 106. The servo MPU 108 executes an operation necessary for the following control process by means of, for example, a digital filter. The digital signal output by the servo MPU 108 is converted into an analog signal by a DA (Digital to Analog) converter 110. The analog signal from the DA converter 110 is amplified by a power amplifier 112, and moves an actuator (VCM: Voice Coil Motor) 114.
The position error signal PE from the position detector 104 is also output to a window comparator 116, which compares it with a threshold value indicating an allowable following error limit. When the level of the position error signal PE exceeds the threshold value in the write operation, a servo error signal is generated, and output to an interface MPU 120 via an AND circuit 118. The servo error signal is then transferred to an upper device 200 by the interface MPU 120. The servo MPU 108 always supervises settling of the head 102 on the basis of the digitized position error signal PE. When the servo MPU 108 determines that the settling has been completed, it outputs a seek completion signal to the interface MPU 120.
FIG. 2 is a waveform diagram showing the conventional settling monitor process. At time t=0, the seek operation has just been completed and the following control process starts. As shown in FIG. 2(a), at t=0, the position error signal PE shows a large position error and is outside of a settling range indicated by hatching. The settling range is selected so that it is narrower than the detection range of the window comparator 116.
FIG. 2(b) shows a position monitor signal OP generated when the level of the position error signal PE is outside of the settling range. The servo MPU 108 has a built-in timer counter formed with hardware or firmware for use in supervision of settling (hereinafter this timer counter is referred to as a settling counter). When the position monitor signal OP is at a high level, the settling counter is continuously preset to a predetermined value which defines a predetermined settling monitor time Tst1. When the level of the position error signal PE enters the settling range, and hence the position monitor signal OP switches to the low level, the settling counter is decremented. When the settling counter value becomes equal to zero, the seek completion signal is generated, as shown in FIG. 2(d). In this manner, it is determined that the settling (seek) operation has been completed when it is detected that the position error signal PE is continuously within the settling range during the predetermined settling monitor time Tst1.
However, in conventional settling monitor systems as described above, there is a possibility that a following control error may occur after it is determined that the settling has been completed. Such a following control error is caused in a process as shown in FIG. 3. The position error signal PE is continuously within the settling range during the settling monitor time Tst1, and hence the condition for generating the seek completion signal is satisfied. However, immediately after this condition is satisfied, the position error signal PE goes out of the settling range.
In order to eliminate this problem, the settling monitor time Tst1 can be lengthened. However, this means increases in the access time, because the seek completion signal is generated at a delayed time.
FIG. 4 is a graph having a horizontal axis showing the settling monitor time Tst1, and a vertical axis showing the error rate (log scale) indicating how frequently following control errors occur. The settling monitor time Tst1 is determined based on an error rate of, for example 10.sup.-12.
It is desired that the settling monitor time Tst1 be short in order to shorten the access time. However, an allowable error rate must be satisfied. Generally, in cases as shown in FIG. 4, a mechanical part which is an object to be controlled has a plurality of resonant structures, which are excited due to a large acceleration during the seek operation. Hence, the cases as shown in FIG. 4 frequently take place under a condition where attenuating vibrations having different frequencies remain and are superimposed.